1. Field of the Invention
This invention relates to liquid pumps and meters and more particularly, but not by way of limitation, to a pump and metering system for pumping and measuring liquid flow in a hot asphalt plant for both the drum mix type and conventional type plants.
2. Description of the Prior Art
By way of background, when asphalt is used for paving projects it is necessary that it be mixed with various amounts of aggregate in order to obtain the proper mixture for its intended use. The mixing and application of asphalt requires that the asphalt be heated. The pumping of asphalt into a mixing plant creates a problem in that unless the hot asphalt is kept continuously moving through the pumping tubes or pipes it will harden and eventually plug up the pipes. Therefore, it is necessary to provide a bi-directional or three-way valve located at the point of mixing and having one of the outlets of the valve being reconnected back to the hot asphalt tank. In this way when the pumping system is pumping asphalt into the mixer and the mixer requires a shut-down of the asphalt being pumped in, it is necessary that this three-way valve be switched so that the asphalt may be continuously pumped back into the asphalt tank so that the hot asphalt will continuously circulate through the pipe. An asphalt plant of this general type is described in the patent to Shearer, U.S. Pat. No. 3,832,201, issued in 1974 and entitled "Process For Making Asphalt Paving Compositions."
It is often the case that the asphalt is purchased from a separate contractor in road building projects and therefore it is necessary to accurately meter or measure the amount of asphalt actually being put into the mixer. Therefore, it is necessary that the metering system be operably connected to the three-way valve in a manner so that it measures the total flow of asphalt therethrough only when the asphalt is actually being moved into the mixer.
The measuring of the amount of asphalt being used also creates a problem since the volume of asphalt per weight or specific gravity varies in accordance with the temperature of the asphalt. Three major types of asphalt, each having different temperature characteristics, have been categorized as (1) emulsified, (2) group zero, and (3) group one. Therefore, a volume metering means, to be accurate, must compensate for the temperature of the specific type of asphalt being measured thereby.
Most systems now on the market utilize a single positive displacement group. Those systems meter directly from the rotation of that pump. The result is acceptable providing there is an adequate supply of asphalt in the tank. However, it is often the case that air gets in the line or the asphalt tank becomes empty and the motor registers more than the actual amount of asphalt delivered into the mixer.
Therefore, in present day systems there are two basic sources of error, the first being the failure to correct the temperature variants which relates to compensation for the type of asphalt being pumped, and the error introduced by air getting into the pumping lines.